Angelus and I were talking on XBL Tuesday night about how many discoverable locations there are in this game and I don't believe either of us was sure at the end of the night. I just discovered Thrond's Altar tonight, and I'm up to 380 now in all three DLC's. I still don't believe that's all of them by any means. I looked in the wikis, IGN and elsewhere and there seems to be no definitive answer. Over 400 seems likely though. I'm only talking about places where you receive a message similar to "Thrond's Altar discovered", which prompts them to register in the stats tab of the pause menu.